mpi_ring.cpp

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#include "ros/ros.h"
#include "mpi.h"
#include <iostream>

int main(int argc, char *argv[])
{
    int next_rank;
    int prev_rank;
    double message;
    int message_tag = 201;

    // Start up MPI

    MPI::Init();
    int rank = MPI::COMM_WORLD.Get_rank();
    int nprocs = MPI::COMM_WORLD.Get_size();
 
    // Calculate the rank of the next process in the ring.  Use the
    // modulus operator so that the last process "wraps around" to
    // rank zero.

    next_rank = (rank + 1) % nprocs;
    prev_rank = (rank + nprocs - 1) % nprocs;

    // If we are the "master" process (i.e., MPI_COMM_WORLD rank 0),
    // put the number of times to go around the ring in the message.

    if (0 == rank) {
        ros::init(argc,argv,"mpi_node",ros::init_options::NoSigintHandler);

        message = 1000;

        ROS_INFO("Process 0 sending %f to %d/(%d processes in ring), with tag=%d",message,rank,next_rank,nprocs,message_tag);
        MPI::COMM_WORLD.Send(&message, 1, MPI::DOUBLE, next_rank, message_tag);
        ROS_INFO("Process 0 sent message(%f) to %d",message,next_rank);
    }

    // Pass the message around the ring.  The exit mechanism works as
    // follows: the message (a positive integer) is passed around the
    // ring.  Each time it passes rank 0, it is decremented.  When
    // each processes receives a message containing a 0 value, it
    // passes the message on to the next process and then quits.  By
    // passing the 0 message first, every process gets the 0 message
    // and can quit normally.

    while (1) {
        MPI::COMM_WORLD.Recv(&message, 1, MPI::DOUBLE, prev_rank, message_tag);

        if (0 == rank) {
            --message;
            ROS_INFO("Process 0 decremented value: %f",message);
        }

        ROS_INFO("Process %d received value: %f",rank,message);
        MPI::COMM_WORLD.Send(&message, 1, MPI::DOUBLE, next_rank, message_tag);
        if (0 == message) {
            ROS_INFO("Process %d exiting",rank);
            break;
        }
    }

    // The last process does one extra send to process 0, which needs
    // to be received before the program can exit */

    if (0 == rank) {
        MPI::COMM_WORLD.Recv(&message, 1, MPI::DOUBLE, prev_rank, message_tag);
    }
    
    // All done

    MPI::Finalize();
    return 0;
}